The similarity of the airless intensely cratered surfaces of Mercury and the moon (Figures 1a and 1b) is in striking contrast to the different densities of the two objects. The similar geologic history inferred from the surface record is surprising, since the different size and density of these two bodies might suggest different amounts of radiogenic heating, silicate/ metal ratios, and thermal conductivity (Figure 2). A conference at the Lunar Science Institute, Houston, Texas, on November 15–17, 1976, focused specifically on such comparisons between Mercury and the moon and attracted 75 attendees representing 35 institutions and a full spectrum of NASA-supported lunar and planetary programs. Many problems concerning the geological evolution of Mercury remain unsolved, but from the papers and discussions at this meeting it is clear that possible solutions require, as a first step, the understanding of all the inner planets. Understanding the detailed history of Mercury would be extremely difficult without the framework developed for the moon. Yet without the data from Mercury the stability of this framework could never be tested. Such iterations represent a fundamental approach for not only solving planetary problems but also formulating new questions.